RFC (round 1), JEP draft: Low-level Object layout introspection methods

Aleksey Shipilev shade at redhat.com
Mon Aug 17 12:33:10 UTC 2020

On 8/16/20 12:41 PM, Peter Levart wrote:
> On 8/11/20 12:22 PM, Aleksey Shipilev wrote:
>> ...but dislike:
>>      public static long addressOf(Object obj);
>>      public static long fieldOffsetOf(Field field);
> What exactly is the purpose of "addressOf" method in terms of 
> information API? Is it used to estimate relative placement of several 
> objects in the heap to see how they are scattered around which affects 
> the CPU cache performance when accessing them?

Yes, it says so in "Motivation" section in JEP. Additionally, checking the object address against
the cache line size.

> If this is the case, then maybe the method could return a "mangled" 
> address: the address + some secret random value calculated once for the 
> whole VM.

Now that is an interesting suggestion!

Implemented here:

There is little-to-none loss of performance, because the offset can be trivially used in intrinsics.
JEP text is updated to mention this technique. I believe this makes the address exposure story less
problematic, although the result is still conceptually a useful proxy for a memory location.


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